At the input node between the CMOS MEMS capacitive sensor structure and the on-chip continuous-time sensing circuit, proper DC bias voltage must be set up and maintained. MOSFET's operated in sub-threshold region have been used to provide the high impedance input bias in the sensing circuits for small capacitive sensors. Sub-threshold transistors can provide simple biasing paths with high impedance while introducing small amount of parasitic capacitance. In the design in L. H. Zhang, G. G. K. Fedder, and L. R. Carley, “A post-CMOS micro-machined lateral accelerometer”, a sub-threshold NMOSFET was used to connect between the input node and the drain terminal of the input NMOSFET of the front-end amplifier in the sensing circuit. Unfortunately, bias instability could exist at the input node. Charging effect due to possible parasitic leakage paths from the input node to positive power supply rail (VDD) could induce a drift of the input node voltage toward VDD and eventually disable the system function. In J. M. Tsai and G. K. Fedder, “Mechanical noise-limited CMOS-MEMS accelerometers”, the design of biasing was improved by using an additional sub-threshold transistor in anti-parallel with the original one and the latch-up phenomenon of input node voltage could be suppressed. Although this arrangement prevents the latch-up problem and retains the simplicity of the original design, the additional transistor introduces more capacitance that degrades signal strength. Furthermore, for both the single-transistor and dual-transistor designs, the mismatch of the sub-threshold transistors imposes significant impact on the output offset voltage of the front-end amplifier. Bias instability has been a critical issue since long time ago.
In P. O'Connor, G. Gramegna, P. Rehak, F. Corsi, and C. Marzocca, “CMOS preamplifier with high linearity and ultra low noise for X-ray spectroscopy”, where the parallel copies of sub-threshold MOSFETs were used for DC biasing the input node of a charge sensitive amplifier, in order to maintain a more constant RDS of the sub-threshold MOSFETs against process, voltage, and temperature variations, an additional complicated self-adaptive circuitry was implemented. The self-adaptive circuitry consumes large chip area and increases power consumption. Furthermore, a current mirror with large scaling ratio was needed in the circuitry, which introduces an additional variation issue. From today's view point, especially for mass production, a reliable and efficient biasing scheme is still in need for practical applications.